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Author Topic: Jitter Specification Input Requested  (Read 9007 times)

Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #45 on: May 04, 2004, 10:00:52 pm »

Zoesch wrote on Wed, 05 May 2004 01:46

 so it's not as if the tools to measure jitter don't exist, manufacturers choose not to use them.


I think it's more complex?  The issues of what and where to measure it, and how to report the results seem the bigger issue.  

Quote:

As to how to report these results... why are we reinventing the wheel? ITU-T and ANSI have very descriptive standards for clocking, wander & jitter performance and synchronization. Failing that, the spectral power density of jitter should be sufficient for 99% of people.


Then we get into the fact that most people can't look at a spectral distribution of jitter and figure out how it's going to manifest itself.  Is 50ns of jitter at 1Hz better or worse than 1ns of noise between 1KHz and 2KHz?  

Then again, most people can't figure out how it will manifest itself because the PLL in the next device will bastardize the clock signal far more than the usefullness of a jitter spec will allow.  Thus, spitting out jitter specs at the BNC output of the box is relatively pointless if the signal then gets upsampled through a series of PLLs in the next box in such a way as to significantly change or add jitter from the first box.  

Quite frankly I'm not disappointed at the fact that the industry, as of yet, doesn't have a jitter spec because adopting a spec suddenly adopts an opportunity to abuse it.  Currently most clock designers (such as Apogee, Lavry, dCS, Aardvark, etc) all avoid specs and say, rather, "ours is good, spec's are unpublishable, you'll have to listen."  And that is pretty much the truth, because the rest of a clock circuit weighs so heavily on the results that it really needs to be a case-by-case evaluation.

Take ANY wordclock source and feed it into an Apogee Big Ben or a Lavry DAC and check out the jitter at the output.  It will be FAR better than taking an Apogee Big Ben or Lavry clock and feeding them into an in-the-computer-PCI-sound-card and measuring the jitter on the converter chip there.  

Nika.
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sfdennis

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Re: Jitter Specification Input Requested
« Reply #46 on: May 05, 2004, 02:05:10 am »

Nika! When and how did you get so cynical?

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Quite frankly I'm not disappointed at the fact that the industry, as of yet, doesn't have a jitter spec because adopting a spec suddenly adopts an opportunity to abuse it.


That's like saying we shouldn't have any laws because people are bound to break them.

Factually, I agree with you: All specs can be 'gamed', and there will always be gamers. But we can't cede the world to the barbarians.

Specs are a good thing provided they are (1) relevant, (2) easy-to-use, and (3) readily verifiable--whether it is for clocks, converters, or anything else. If the specifications for a devices such as clocks aren't relevant beyond some minimum performance bar, than all the industry needs is a quick pass/fail test, and no specs are necessary at all. They're irrelevant. If they're not easy to use, no one will know what to do with them. If it takes a $40K piece of equipment and 6 months training to validate the device, well that's too hard and unscrupulous manufacturers will cheat.

Every one of these three aspects has been raised in this thread, and the discussion can be applied to specs for any device.

Clocks and PLLs are either important or they're not. Even if they are important, clocks might be a binary category with little room for differentiation: either a device is 'good enough' or it is crap. If this is so, then there will be no opportunity for competitive performance differentiation in the 'good enough' group. One good-enough clock will be just like any other. They will become commodities prices will fall, and manufacturers will lose their margins. Devices in the crap group will be outed by the market and won't survive. You can see why manufacturers might not want specs. Don't give in.

It is sad that in general, specs for audio equipment have a bad rap. I have a pet theory about this. Some of it results from the gamesmanship of godless manufacturers. But I think that the majority of it, and the resulting cynicism, stems from consumer's inability to receive provided specs and translate those specs into a specific expectation of an audio experience.

When I see a THD+N curve for an amplifier, I have a pretty good feeling of how certain passages of my favorite recording of Madama Butterfly are going to sound through it. Over time, I've been less and less surprised. When I am surprised, I will take some time to figure out why. You'd be amazed at how often circuits in demo stations are improperly wired (usually both channels left or right) or speaker elements and channels are downright fried. Every now and then it is the actual amplifier. By being surprised time and again, my ability to 'hear the spec' has improved. The process isn't too distant from learning to read music: see the notes, hear 'em in your head, play 'em.

We should encourage this learning feedback loop by creating relevant, easy-to-use, readily verifiable specs. I think that it is important work that thoughtful folks like you, Zoesch, Ethan, and others here can contribute to. So don't be discouraged.

Sorry for the rant.

-Dennis

ps. Thanks, Zoesch, for the tip on NOT using quotes in Word. -D
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Dennis Tabuena

Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #47 on: May 05, 2004, 10:15:28 am »

Dennis Tabuena wrote on Wed, 05 May 2004 07:05

Clocks and PLLs are either important or they're not.


Precisely, but specs on one are relatively useless without specs on the other.  The clocking of a system is exactly that - it is a comprehensive system, and cherrypicking one device from the system to measure is unto itself relatively useless information.  Because it is useless it inherently allows for a degree of bogosity - even formalizing a spec becomes bogus if it provides irrelevant information.  Until we can have some sort of spec on a clocking system that indicates what the jitter in the clock is that gets into the A/D chip then we don't have a useful specification at all.  I contend that so far the discussion on measuring jitter at the clock source does not get us closer to the jitter at the A/D chip, primarily because the eventual spec will be so misunderstood as to render it essentially useless, on top of the fact that it is relatively useless anyway in that it isolates a single component in an interactive and comprehensive system.


Nika.
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sfdennis

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Re: Jitter Specification Input Requested
« Reply #48 on: May 05, 2004, 11:20:49 am »

Nika Aldrich wrote on Wed, 05 May 2004 07:15

...but specs on one are relatively useless without specs on the other.  The clocking of a system is exactly that - it is a comprehensive system, and cherrypicking one device from the system to measure is unto itself relatively useless information.


If you really believe that, then when do you stop?

One could say that the system is the entire recording chain, and that's clearly ridiculous. It is akin to saying mic specs aren't relevant because how they sound depends on the preamp you use with them. Well it is true that the preamp shapes the sound, but geez, those are spec'd too. Having specs for both gives us an opportunity to identify how these things are going to translate our recordings.

If you put a crappy clock--say one that has a propensity to randomly and abruptly jump from one frequency to another far-way frequency, well there ain't no PLL that will be able to deal with that.

Conversely if you put a well conditioned signal from a cesium clock into a converter with an improperly damped PLL, well you're hosed anyway.

It is useful to spec both. Spec the stability of the clocks. Spec the jitter attenuation of the PLLs in their locking state as well as their lock-time. Also spec their stability in their flywheeling state (in this state, they're their own clock).

And yes, you can't take a direct measurement of jitter inside the chip, but if you know the quality of the incoming clock--whether it is from an external source or inside the converter--then you have some basis for identifying the weak link in the chain. And that is very useful.

-Dennis
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Dennis Tabuena

Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #49 on: May 05, 2004, 11:31:16 am »

Dennis Tabuena wrote on Wed, 05 May 2004 16:20

If you really believe that, then when do you stop?


When the quality of a single device is not overwhelmingly obfuscated by the quality of another, unspecified device that it is dependant upon, such that it is with clocks.

Quote:

It is useful to spec both. Spec the stability of the clocks. Spec the jitter attenuation of the PLLs in their locking state as well as their lock-time. Also spec their stability in their flywheeling state (in this state, they're their own clock).


And until we do both, and until the specs are meaningful for both then doing only one of them is relatively meaningful,  yes?

Nika.
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